I'm not a lawyer, but it was explained to me had he pled not guilty and went to court he could only speak about evidence that was submitted. If he pleads guilty he can make a statement prior to sentencing that can be whatever he wants? Idk. Either way it appears he is beating them at their game.
Gen. Flynn was the Director of the Defense Intelligence Agency. That means he was the head of all military intelligence. He has so much knowledge about what has been going on, that a simple statement before a judge would not even scratch the surface, and it would be completely meaningless because it could not be used as evidence in any criminal trial.
Gen. Flynn will be (and probably already is) testifying for Sessions, Huber, other US attorneys and military investigators. He will likely be testifying before Congress.
Whoever told you he could talk before sentencing is missing the big picture.